The present invention relates to a method of producing seamless steel tube, particularly a method of producing seamless steel tube from billet having a square or rectangular cross-section by using a press roll piercer.
Conventionally and generally the production of seamless steel tube is carried out mainly by subjecting round billet to piercing by a Mannesman piercer; rolling the billet into a hollow shell by an elongator; and finishing by a plug mill. Such production methods use round billets as the starting material, and the supply of such billets requires a special continuous casting system, which constitutes one of the problems in the effort to reduce material cost.
In addition, the piercing step using the Mannesman piercer operates under such conditions as a 5-10% expansion percentage and a percent 1.5 - 2.5 elongation ratio, making it difficult to apply to alloy steel. Therefore, if it is used as the material for said step, the round billet of carbon steel should be free from surface defects, inclusions and any other irregularities.
After having been pierced by the piercer, the hollow shell is expanded and elongated by the elongator of the same type as the piercer. Generally in such an operation, the elongation ratio and the expansion ratio are respectively 2 - 3 and 10 - 20%; therefore, any non-uniformity in the wall thickness of the hollow shell resulting from the previous step may be increased by the operation of the elongator.
Particularly in the case of producing a hollow shell of very small wall thickness, the increased rolling load at the elongator makes it generally necessary to reduce the slant angle of the rolls, resulting in a rolling speed of 0.2 - 0.3/sec in the axial direction. This means that the temperature drop in this step is so great as to require a reheating process before the rolling by the plug mill.
In addition to the abovementioned seamless steel tube production process, there is a process practised by using a Pilger mill. According to this process, a piece of square or rectangular cross-section material (mostly ingots) is inserted into a container, and subjected to press piercing by an Ehrhardt piercing process as it is inserted; then it is roughed in the abovementioned elongator and finished in the Pilger mill. However, this process is greatly inferior to the plug mill process with respect to rolling efficiency, preciseness of dimensions and surface quality.
When round billet is used as the material for steel tube production, the material cost increases as described above. This problem has apparently been solved by a recent improvement of the above-described press process, according to which part of the container is replaced by rolls, and which is named the press roll piercing process, thereby making it possible to produce round hollow shell from a billet of square or rectangular cross-section. However, even this process is not free of the problem of the non-uniform wall thickness of the hollow shell in the circumferential direction.